For the purposes of this application, a bond between two substrates or container laps is the mechanism that holds the substrates together and resists the separation of the substrates when subjected to forces perpendicular to the plane of the substrate. Bond strength is the ultimate separation force that the bond can resist after the adhesive is set or cured. Green strength is the ultimate separation force that the bond can resist at a given time prior to final curing or setting of the adhesive.
In the field of closing containers such as boxes, cases, cartons, etc., with an adhesive, it is important to limit open time in order to avoid drying or setting up of the adhesive before the flaps are closed. At the same time, the adhesive must provide sufficient early strength, or green strength, shortly after the flaps are closed in order to prevent popping apart once the closing force has been removed.
Adhesives used in closing containers vary in final bonding strength and in green strength. For instance, a cold glue adhesive generally requires about 10-30 seconds to achieve sufficient early strength to prevent popping apart of the flaps. A typical hot melt adhesive requires under three seconds and sometimes less than one second in order to achieve sufficient early strength.
Primarily due to greater early strength, hot melt adhesives are generally preferred over cold glue adhesives for assembly line closing of containers. However, under some circumstances a cold glue adhesive may be substituted for a hot melt adhesive if it is known that the substituted adhesive can provide sufficient early strength within the prescribed time limit. To determine whether or not a particular adhesive would be suitable for a desired purpose, and to enable optimization of assembly line container closing, it is desirable to compare the bonding properties, particularly the green strength, of various adhesives on an accurate and repeatable basis.
Present bond testers provide some useful data for comparing the performance properties of adhesives by measuring the force required to pull apart two adhesively bonded substrates. Two well known bond testers measure the strength required to break a bond between two substrates when the bond is placed in tension under a constant force. A strain gauge or an equivalent measuring device provides an indication of the bond's resistance to the applied constant force at the point in time when the bond fails.
Although a bond tester of this type can provide comparative data of some value in assessing relative bond strength when the measured strengths of the adhesive are fairly close, such a bond tester does not provide sufficient comparative data for adhesives which have a wide range in bond strength. An adhesive bond under tension will initially resist the force of tension to some degree, thereby slowing the rate at which the substrates are pulled apart to break the bond. Therefore, despite the application of a constant pull force, the rate at which the substrates are pulled apart varies appreciably. As the difference in bond strengths between two adhesive bonds increases, the difference in the rates at which the substrates are pulled apart to break the bonds also increases.
Thus, while some indication of the strength of a bond can be obtained with a bond tester of this type, the obtained value is not particularly useful for comparative purposes. In short, the application of a constant force to break an adhesive bond does not in and of itself provide adequate comparative performance data for evaluating the relative bond strengths of adhesives on an equal basis. Moreover, the bond testers of this type do not provide any measure of the total work expended during tension to break the bond.
In another type of bond tester, two substrates are bonded together with an adhesive at a first location. After a sufficient time has elapsed for the bond to achieve final strength, the bonded substrates are transferred to a second location whereupon the bond strength of the bond is measured while moving a wire rod laterally at a constant rate of speed between the two substrates to effect a failure of the bond.
Although a device of this type may be used to obtain comparable final strength measurements for various adhesives, it is not suitable for use in obtaining measurements of the green strength of an adhesive bond, i.e., the strength of the bond shortly after compression is applied but before final strength is achieved. A device of this type does not provide data sufficient to indicate whether or not a particular adhesive will provide a bond of sufficient strength within a prescribed short time limit. Moreover, this type of device does not measure the tensile strength of the bond, the force required to pull the bond directly apart. It is more in the nature of a peel tester, testing the peel strength of an adhesive bond.